Revised by Joey Williamson, HGIC Horticulture Extension Agent, Clemson University, 10/15. Originally prepared by Marjan Kluepfel, HGIC Horticulture Specialist, and Bob Polomski, Extension Consumer Horticulturist, Clemson University. New 05/99. Images added 04/09.
Gardenias have been popular shrubs in South Carolina since the 18th Century, and have been grown by the Chinese for over a thousand years. They were named after the Scottish naturalist Alexander Garden (1730 – 1791). Gardenias are not the easiest shrubs to grow, but the exquisite white, fragrant flowers make up for the extra attention gardenias require.
Double-flowered gardenia (Gardenia jasminoides) blossom.
Karen Russ, ©2008 HGIC, Clemson Extension
Gardenia (Gardenia jasminoides) is also known as cape jasmine, and is an evergreen shrub that typically grows to a height of 3 to 8 feet, depending upon the cultivar. Spread is usually about the same as the height. The foliage of well-fed shrubs is glossy, dark-green, 2 to 4 inches long and half as wide. Depending on the cultivar, the flowers can be either single or double and from 2 to 4 inches in diameter. They are waxy, white and very fragrant.
The growth rate is medium.
Gardenias are primarily grown for their fragrant flowers and handsome foliage. They should be planted where people will notice the fragrance. The flowers open over a long period of time, from May through June, and sporadically throughout the summer. Gardenias are considered deer resistant.
Single flowered gardenia blossom (Gardenia jasminoides).
Flickr: Creative Commons License 2.0
Gardenias require considerable maintenance. Fall is the best time for planting. They are best planted in light to partial shade; preferably with morning sun and afternoon shade. Gardenias resent root disturbance. Smaller cultivars will also grow well in containers and placed where their fragrance can be enjoyed.
Gardenias prefer acid (with a pH of less than 6.0), moist, well-drained soils. Add organic matter, such as compost or ground composted pine bark, to the planting bed and till into the soil before planting. Mulch plants with a 2 to 3-inch deep layer of pine straw, compost or ground bark.
Fertilize gardenias lightly in the spring once frost has passed with a well-balanced, extended-release, acid-forming, azalea fertilizer. Fertilize the shrubs again 6 weeks later to encourage extra flowers or faster growth of young shrubs. By well-balanced, this means to look for nutrients in the ratio of 2-1-1. Fertilizer examples are:
Complete, acid-forming organic fertilizers are also excellent choices for use on gardenias, for spring and early summer fertilization. They are typically not as nutrient rich, and because of both the low nitrogen content and their inability to burn the roots, they can be mixed lightly into the soil in the fall at planting to enhance root growth. Organic fertilizer examples are:
Do not fertilize gardenias in the fall. Doing so will stimulate tender growth, which may be killed if the temperature in winter drops below 15 degrees. Gardenias are cold-sensitive and during severe winters can be killed to the ground in the Upstate. Often they regenerate in spring.
Additional products containing iron may be applied during the growing season, if needed to correct the yellowed new foliage caused by an iron deficiency. This may occur if gardenias are limed or are planted near a new concrete foundation. An example of an iron-supplement is Ironite Mineral Supplement, which contains not only iron, but also numerous trace elements.
Gardenia “hips” are the fruit that occasionally appear in fall (Gardenia jasminoides).
Karen Russ, ©2008 HGIC, Clemson Extension
Prune shrubs after they have finished flowering in summer to remove straggly branches and faded flowers. Gardenias should be watered weekly during periods of drought in summer. Drip-irrigating the shrubs will keep water off the foliage and blossoms and prevents leaf spots.
Most of the older gardenia cultivars are cold hardy to USDA zone 8, but many of the newer and smaller cultivars are hardy to at least USDA zone 7a. Dwarf cultivars, however, are often more cold sensitive.
A whitefly infestation is the most commonly occurring problem on gardenias anywhere in the state. Whiteflies have piercing-sucking mouthparts, with which they penetrate the cells of a leaf, and then suck out the leaf sap. The top sides of infested leaves may become pale or spotted. These small, white-colored flies often remain unnoticed, as they primarily infest the lower surface of each leaf. As this pest removes plant sap, it excretes a large amount of clear, colorless, sugary waste, which drips onto the leaves below. This sugary waste, called honeydew, is quickly colonized by a black mold (sooty mold), which coats the leaves in summer.
Whiteflies may be controlled by sprays to the lower surfaces of leaves with an insecticidal soap solution or a horticultural oil mix. Both of these less toxic insecticides kill by suffocation. Sprays may need to be repeated every few days until the whitefly population is under control. Follow label directions for mixing an insecticidal soap spray. Use horticultural oils as sprays between the temperatures of 45 and 90 degrees, and spray in the early morning or late evening to slow the drying time of spray. For horticultural oil, mix and spray a 2% solution (5 tablespoons of oil per gallon of water). See Table 1 for examples of brands and products.
Dark-colored sooty mold will grow on the sticky honeydew, which drips from whiteflies when feeding on gardenia foliage.
Joey Williamson, ©2015 HGIC, Clemson Extension
Alternatively, numerous synthetic pyrethroid contact insecticides (such as bifenthrin, cyfluthrin and lambda cyhalothrin) will also control whiteflies if sprayed on the lower surfaces of leaves. Acephate, however, is a foliar systemic insecticide that will move through the leaf tissue. So sprays to the upper leaf surfaces will move downward to control the whiteflies. All foliar sprays may need to be repeated once or twice at 10 day intervals, as they typically do not kill the eggs. Do not spray plants in bloom to prevent injury to pollinating insects. See Table 1 for examples of brands and products.
Instead of numerous insecticidal sprays, a single soil drench of imidacloprid can be applied at the base of the shrubs in the spring as new growth appears to give season long control. Follow label directions for mixing and application. See Table 1 for examples of brands and products.
Similar in habit are small gray aphids, which cling to leaf undersides. These can be easily controlled with insecticidal soap sprays. Nematodes are microscopic worms, which live in the soil and feed on plant roots. In sandy soil, these plant parasitic nematodes can cause gardenias to become stunted or even die. Root rots caused by several different fungi can also be a problem, primarily in poorly drained soils.
Although much less common, another problem encountered is "bud drop." Flower buds may abort and drop off just before they open. Causes of bud drop include low humidity, overwatering, under-watering, insufficient light and high temperatures (night temperatures between 50 and 55 °F are necessary for the formation of flower buds).
Larger, Upright, Double-flowered Cultivars:
Smaller Cultivars with Repeat Blooming:
|Active Ingredients||Examples of Brands & Products|
|RTS = Ready to Spray (a hose-end spray bottle)|
|Acephate||Bonide Systemic Insect Control Concentrate|
|Bifenthrin||Ferti-lome Broad Spectrum Insecticide Concentrate; & RTS
Hi-Yield Bug Blaster Bifenthrin 2.4 Concentrate; & RTS
Up-Star Gold Insecticide Concentrate
Bifen I/T Concentrate
Talstar P Concentrate
|Cyfluthrin||Bayer Advanced Vegetable & Garden Insect Spray Concentrate; & RTS|
|Horticultural Oil||Ferti-lome Horticultural Oil Spray Concentrate; & RTS
Southern Ag Parafine Horticultural Oil
Bonide All Seasons Spray Oil
Lilly Miller Superior Type Spray Oil Concentrate
Monterey Horticultural Oil Concentrate
|Imidacloprid||Bayer Advanced Garden 12 Month Tree & Shrub Insect Control Concentrate Landscape Formula
Bayer Advanced 12 Month Tree & Shrub Insect Control Protect & Feed (2-1-1)
Bonide Annual Tree & Shrub Insect Control with Systemaxx
Ferti-lome Tree & Shrub Systemic Insect Drench
Ferti-lome Tree & Shrub Systemic Insect Granules
Green Light Tree & Shrub Systemic Insect Killer Concentrate; & Granules
Gordon’s ImidiPro Systemic Insecticide
Gordon’s Tree & Shrub Insect Killer
Monterey Once A Year Insect Control II
Bonide Borer Miner Killer RTS
Bonide Systemic Insect Spray w/ Systemaxx RTS
Ferti-lome Systemic Insect Spray Concentrate; & RTS
Hi-Yield Systemic Insect Spray Concentrate; & RTS
Spectracide Tree & Shrub Insect Spray Concentrate; & RTS
Merit 2 Granular
Martin’s Dominion Tree & Shrub
|Insecticidal Soap||Bonide Insecticidal Soap Concentrate
Espoma Earth-tone Insecticidal Soap Concentrate
Natural Guard Insecticidal Soap Concentrate
Schultz Garden Safe Insecticidal Soap Insect Killer Concentrate
Safer Brand Insect Killing Soap Concentrate
|Lambda Cyhalothrin||Spectracide Triazicide Insect Killer for Lawns & Landscapes Concentrate; & RTS|
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This information is supplied with the understanding that no discrimination is intended and no endorsement of brand names or registered trademarks by the Clemson University Cooperative Extension Service is implied, nor is any discrimination intended by the exclusion of products or manufacturers not named. All recommendations are for South Carolina conditions and may not apply to other areas. Use pesticides only according to the directions on the label. All recommendations for pesticide use are for South Carolina only and were legal at the time of publication, but the status of registration and use patterns are subject to change by action of state and federal regulatory agencies. Follow all directions, precautions and restrictions that are listed.